Most of the information on these pages is here for my benefit, i.e. there was something I wanted/needed to know, so I gathered that information. I have posted it on the chance that it might be useful to someone else.

Motor Stuff

You can calculate the thrust to weight ratio of a motor using the equation:

Thrust to weight ratio = Motor's average thrust / 4.45 / ratio

Motor Thrust:

To get the accepted 5:1 thrust to weight ratio of a J330 you divide 330 by 4.45, (the number of Newtons per pound). Divide the result by 5 to get 14.83 Lbs., which is the 5:1 thrust to weight ratio.

If you have built a rocket before choosing an ideal motor, you can also find out what motors will be OK to fly in your rocket by using this equation:

Motor's average thrust = Rocket weight in Lbs * 4.45 * ratio

Rocket Weight:

So if your rocket weighs in at 15 Lbs. then multiply it by 4.45 and then again by the ratio you want and you'll get a number you can compare to the average thrust of a motor. Any motor with an average thrust greater than the value calculated should be fine.

Descent Rates and Parachute Sizing

I use a fairly well-published descent calculation:

D = sqrt( (8 m g) / (pr Cd v2) )

Where:

D is the chute diameter in meters

m is the rocket mass in kilograms

g is the acceleration of gravity = 9.8 m/s2

p is 3.14159265359

r is the density of air = 1.22 kg/m3

Cd for the chute, which is 0.75 for a flat sheet parachute, or 1.5 for a hemispherical parachute.

v = ground impact speed

Rocket should hit the ground at ~15 fps. 16.4 fps equals a drop from 4'. Drogue chutes descent rates should be around 50 fps.

1 pound = 0.45359237 kilogram

Rocket Weight (kg):

Example

We'll size a parachute for my RCDP Flash-M, which weighs in at 17.5 pounds with 75/6400 case, or 7.955 kg.